Attempts have been made in the past to identify the range and/or altitude of a large number of electromagnetic radiating targets quickly and to a high degree of accuracy, precision and resolution. Preferably, the range and/or altitude is established by a passive method, that is a method which does not require the locator to emit an electromagnetic signal. Previous attempts to provide such systems include techniques known as the Two Platform Cooperative Location technique, the Self Passive Location System, the Passive Ranging On Scanning Emitters technique (PROSE), the Passive Ranging On Non-Scanning Emitters technique (PRONSE), and the Passive Rotating Receiver Ranging technique (PR.sup.3).
The Two-Platform technique operates well in connection with a single emitting target, but when more than two emitting targets are within range of such a system, the system gives a set of target locations and ghosts equal to the square of the number of emitters. Although deghosting techniques are available to eliminate the ghost or false locations, these techniques require a long time and considerable computer capacity to implement. The Self Passive Location technique uses a single receiver platform, but only works on emitters which maintain a constant heading and velocity for a 10 to 15 minute period.
The PROSE system is effective only on targets which emit a scanning signal and whose scan rate is known. The PRONSE technique solves this problem, but requires two or more widely separated sensors. Although such sensors may physically be put on each wingtip of an airborne fuselage, it is necessary to know the location and altitude of each sensor with respect to one another with a great degree of precision. The flexibility of large structures, such as an airborne fuselage or wings, mandates the use of an auxiliary locating system, such as an inertial or laser system, to track the relative movement of the sensors. The PR.sup.3 system has the same problems as the PRONSE system. In addition, since the PR.sup.3 system involves a rotating receiver, there is a requirement of knowing the pointing angle of each sensor to a precision of greater than one part per million.
The subject invention improves upon a known prior art technique referred to as a multi-path ranging system or a Time Delay Of Arrival (TDOA) System. A TDOA system or technique relies upon the fact that the surface of the earth or sea will reflect electromagnetic waves emitted from a target. For example, in FIG. 1 there is illustrated a target 10 which emits electromagnetic waves and a receiver 12 which is capable of receiving those waves. When target 10 emits an electromagnetic signal, receiver 12 detects a wave train consisting of the sum of signals traveling the direct path R.sub.S between target 10 and receiver 12 and signals traveling by means of a reflection path comprising R.sub.R1, ground bounce point 14, and R.sub.R2. The difference in transit time .DELTA.T.sub.1 between the direct and reflected waves is determined by finding the differential time associated with the non-zero maxima of the autocorrellagram of the received time history. When this time difference .DELTA.T.sub.1 is multiplied by the velocity of a electromagnetic propagation (.upsilon.), the difference in path length .DELTA.R is known (.DELTA.R=.DELTA.T.sub.1 .multidot..upsilon.). This information, the altitude (R.sub.H) of the receiver 12, and the depression angle (.alpha..sub.D) of target 10 may be used to determine the range R of target 10 in accordance with the following equation: EQU R=[(R.sub.H).sup.2 -(.DELTA.R/2).sup.2 ].multidot.cos (.alpha..sub.D)/[(.DELTA.R/2)-(R.sub.H).multidot.sin (.alpha..sub.D)](1);
where:
R is the range between target 10 and receiver 12; PA1 R.sub.H is the altitude of receiver 12; PA1 .DELTA.R is the difference between the length of the direct path (R.sub.S) and the reflection path (R.sub.R1 +R.sub.R2); and PA1 .alpha..sub.D is the target 10 depression angle. PA1 R=[(R.sub.H).sup.2 -(.DELTA.T.sub.1 .multidot..nu./2).sup.2 ].multidot.cos (.alpha..sub.D)/[.DELTA.T.sub.1 .multidot..nu./2-R.sub.H .multidot.sin (.alpha..sub.D)] PA1 .nu.=the speed of said signals; and PA1 .alpha..sub.D =tan.sup.-1 [sin (.DELTA.T.sub.2 .multidot..tau.) /tan (.theta.)].
Once the range R has been established, the target altitude (T.sub.H) is: EQU T.sub.H =R.sub.H -R.multidot.tan (.alpha..sub.D) (2).
It should be understood that the above equations are only approximate and that corrections can be added to account for the curvature of the earth.
Range and altitude can also be determined by using the path length difference and other angles, such as the depression angle between the direct and reflected paths. The major problem with such prior art multipath ranging systems lies in the accuracy required in measuring the depression angle used. For example, an error of less than 0.1 degrees of arc in depression angle .alpha..sub.D produces a range error exceeding 10 percent.
Accordingly, it is an object of the subject invention to provide a system and related methods which permit the passive location of a large number of electromagnetic radiating targets to be determined quickly and to a high degree of accuracy, precision and resolution.
It is another object of the subject invention which overcomes the disadvantages of inaccurate depression angle measurement found in prior art multipath or TDOA systems.
Additional objects and advantages of the invention will be set forth in the description which follows and, in part, will be obvious from the description or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.